Download Download Chapter 6: Conclusions

CHAPTER SIX:
CONCLUSIONS
Aaron B. Shiels
Department of Botany
University of Hawaii at Manoa
3190 Maile Way
Honolulu, HI. 96822
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Along with humans, introduced rats (Rattus rattus, R. norvegicus, and R. exulans)
and mice (Mus musculus) are among the most invasive and widely distributed mammals
on the planet; they occur on more than 80% of the world‘s islands groups (Atkinson
1985; Towns 2009). By incorporating modern technology, such as aerial broadcast of
rodenticides, the number of islands where invasive rodents can be successfully removed
has recently increased (Howald et al. 2007). However, successful rat and mouse
eradication on relatively large (> 5000 ha) or human-inhabited islands such as the main
Hawaiian Islands rarely occurs (Howald et al. 2007) despite large sums of money and
research efforts annually to combat invasive rodent problems (see Chapter 1 section ―Rat
history in Hawaii‖; Tobin et al. 1990). Therefore, it is highly unlikely that invasive rats
and mice will be eradicated from relatively large, human-occupied islands such as Oahu
in the near or distant future (Howald et al. 2007); and accepting this may be a first step
towards increasing the likelihood of native species conservation in archipelagos like
Hawaii where introduced rodents have established.
Determining which invasive rodent species are present at a given site is important
because the risks that some rodent species pose to particular (prey) species and/or
habitats differ from those posed by other rodent species. Two sympatric species cannot
occupy the same niche indefinitely, in a stable environment (Gause 1934), which may
partly explain why some rodent species may not occur where others are present (Harper
2006). For example, in New Zealand, mice rarely occur with Pacific rats (Yom-Tov et al.
1999; Ruscoe and Murphy 2005). The behavior and abundance of rodent species may
also change when they are sympatric with other rodent species (Grant 1972; Yom-Tov et
al. 1999; Russell and Clout 2004). Therefore, determining rodent species composition,
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relative abundances, diets, and habitat uses are critical for understanding how rodent
species may coexist in a given environment and the relative impacts that each rodent
species may have upon native and non-native biota. The following questions,
hypotheses, and findings from my study relating to rodent species composition, relative
abundances, diets, habitat uses, fruit removal, and seed fates, are summarized below.
Question I: How pervasive are rats and mice in Hawaiian mesic forests? What are
their distributions, habitat uses, and seasonal fluctuations?
Hypothesis 1: The abundance of coexisting rodent species is highest for the dominant
rodent, the black rat, second highest for mice, and lowest for Pacific rats.
Findings: This hypothesis was supported. The black rat (7.1 indiv./ha) was the most
abundant rodent at all three sites when averaged across all sampling periods, whereas the
house mouse (3.7 indiv./ha) was second highest, and the lowest abundance was for the
Pacific rat (0.3 indiv./ha; Chapter 2). The period that tended to have the highest black rat
abundance was April-May and October-December; there did not appear to be distinct
months where Pacific rat and mouse abundances were consistently elevated among sites,
but within a site there were fluctuations across sampling months for each species‘
abundance. Although most studies do not report abundances of more than two introduced
rodent species, there are a few studies that have found similar patterns as my study for
insular forest habitats (Russell and Clout 2004; Harper 2006; Harper and Cabrera 2010),
including those in Hawaii (Tamarin and Malecha 1971; Sugihara 1997; Lindsey et al.
1999). However, some studies in Hawaii have found different patterns of relative
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abundances for coexisting rodents, such as in high elevation studies where mice are more
abundant than black rats (Amarasekare 1994; Banko et al. 2002), or at low elevation wet
forest where abundances of Pacific rats are slightly higher or similar to black rats (Beard
and Pitt 2006).
Hypothesis 2: The location of each rodent species’ nest/den site, and activity, differs
among rodent species: Black rats den and are more active in trees than on the ground,
whereas Pacific rats and mice den in, and are more active on, the ground than in trees.
Findings: This hypothesis was supported for my studies in the Waianae Mountains. As
evidenced by spool-and-line tracking, all of the monitored Pacific rats and all but one of
the monitored mice had dens belowground. Additionally from radio-tracking, all five of
the radio-collared Pacific rats denned belowground. Black rats commonly denned aboveand belowground, as evidenced by radio-tracking and spool-and-line tracking. For the 24
black rats radio-collared and followed at the two sites (KHI and MAK) in my study, ca.
42% had den sites both in the ground and in trees; whereas ca. 43% denned only in trees
and ca. 15% denned only in the ground. Using spool-and-line tracking, 25-59% of the
monitored black rats had den sites in the trees. In contrast to my findings from Oahu,
most studies elsewhere found that black rats den only in trees (Dowding and Murphy
1994; Hooker and Innes 1995; Lindsey et al. 1999). Studies of the locations of den sites
for Pacific rats and mice are much less common than those for black rats; however,
Lindsey et al. (1999) found that all three Pacific rats in Hawaiian wet forest had
belowground den sites. There were no studies found describing the locations of den sites
for the house mouse.
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The Pacific rat and house mouse used mainly ground surface habitat when
monitored by spool-and-line tracking; 70% and 69% of the movements recorded for
Pacific rats and mice, respectively, were on the surface. In contrast, 32% of the
movements for black rats were on the surface, and the majority (64%) of black rat
movement was recorded aboveground at heights of ca. 2.8 m. These findings are similar
to those of Hooker and Innes (1995) in New Zealand forest where black rats were
primarily arboreal and 73% of the radio-tracking locations were > 2 m aboveground.
Lindsey et al. (1999) found that 43 of 44 of the rats they trapped in trees were black rats,
and only one Pacific rat was caught in a tree in their 2 year study. Hoare et al. (2007)
concluded that Pacific rats on a New Zealand off-shore island were most active on the
surface (85% of recorded movements) relative to aboveground (10%). There have been
no studies that I know of that track mice to determine their uses of surface, above-, and
belowground habitat; however, most studies attempt to trap mice only on the ground
surface (Dickman 1992; King et al. 1996; Sugihara et al. 1997; Arthur et al. 2005; Ruscoe
and Murphy 2005; Harper 2010).
Question II: What are the diets of rats and mice in Hawaiian mesic forest?
Hypothesis 3: The diets of introduced rodents differ by species in the following pattern:
diets of black rats are dominated by plant material, those of mice by animals, and those
of Pacific rats by nearly equal proportions of plant and animal material.
Findings: This hypothesis was supported for my study at Kahanahaiki (KHI) as
evidenced by stomach content analysis. Black rat stomach contents were dominated by
plant material (81% relative abundance), and secondarily by arthropods (14%). Mouse
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stomach contents were dominated by arthropods (57%) rather than plants (36%). The
diet of the Pacific rat fell between that of the black rat and the house mouse, as evidenced
by Pacific rat stomachs containing 60% plants and 38% arthropods (Chapter 3). These
findings are similar to those of Sugihara (1997) from wet forest, where black rats ate
more fruits, seeds, and other vegetative material than did Pacific rats. However, several
studies in New Zealand have shown that the dominant food type in black rat stomachs is
arthropods, particularly weta (Innes 1979; Gales 1982; Miller and Miller 1995). In
contrast to these New Zealand studies, Beard and Pitt (2006) from lowland wet forest in
Hawaii found that black rat and Pacific rat stomachs contained 99% plant material.
Pacific rat diets on 37 Tokelau islands were dominated by plant material (88%; mostly
coconut), and contained just 4% arthropods (Mosby et al. 1973). Following a review of
studies from Southern Atlantic islands that have been invaded by mice, Angel et al.
(2008) concluded that arthropods are the dominant food type in mouse diets. However,
the one other Hawaiian study that examined stomach contents of mice occurred in highelevation shrubland and the mouse diets were plant-dominated (63%; particularly seeds
and vegetative material), and contained just 33% arthropods (Cole et al. 2000). It is
important to note that the types of food consumed by rodents largely depend upon the
types that are available when sampling occurs, as well as a suite of other factors including
the richness of rodent species, the chemical and nutritional content of available foods,
habitat characteristics, and seasonality (Clark 1982; Harper 2006).
The long-term (lifetime) diets of each of the three introduced rodent species were
determined from individuals sampled at KHI through stable isotope analysis (δ15N and
δ13C) of bone marrow. Although the stomach content analysis revealed that Pacific rats
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had an intermediate diet between the more carnivorous house mouse and the more
vegetarian black rat, the lifetime diet of the Pacific rat, as revealed by stable isotopes, was
nearly indistinguishable from the lifetime diet of the house mouse. There are several
possible explanations for the difference between the stomach content and stable isotope
findings for Pacific rats, including the similar amounts of caterpillar and/or grass seeds
consumed by both mice and Pacific rats, similar foraging microsites for mice and Pacific
rats (Chapter 2; Stapp 2002), the small sample size of Pacific rat stomachs analyzed (N =
12), or the assimilation rates of the different types of prey among rodent species (Gannes
et al. 1997).
Question III: Which of the dominant species in Hawaiian mesic forest have their
diaspores (fruits + seeds) removed from the forest floor by rats and mice? What are
the fates of such rodent-handled seeds?
Hypothesis 4: Vulnerability to diaspore harvest and seed predation by rats is more
strongly associated with diaspore size (mass and greatest diameter) than other
characteristics.
Findings: This hypothesis was not supported. Diaspore (fruit + seed) and seed size
often influences whether a seed is found by a rodent, removed or consumed in place, and
destroyed or discarded intact (Cowan 1992; Izhaki et al. 1995; Williams et al. 2000). For
a number of reasons including those associated with optimal foraging (Smith and
Reichman 1984), I expected that larger seeds would be harvested, consumed, and
destroyed by rodents more commonly than smaller seeds were (Thompson 1987; Crawley
1992; Hulme 1998). Contradicting my expectation, I found that the largest diaspore with
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the largest seed of all species studied (Aleurites moluccana) was not harvested (i.e., 0%
removal) from the forest floor, and when offered to black rats in captivity the fruit and
seeds were rarely eaten (> 97% of fruit and seed mass was uneaten) and the seeds never
destroyed. Similarly, the next two largest seeds examined (Pouteria sandwicensis and
Sapindus oahuensis) were rarely consumed and most (> 85%) survived in captive feeding
trials despite 88% of the P. sandwicensis diaspores harvested by black rats in the field.
Therefore, the likelihood of diaspore harvest from the forest floor, and the likelihood for
seed predation, did not appear to be size dependent (Chapter 4). Overall, the largest and
smallest seeds tested in the field experiment (Chapter 4) had the highest survival when
offered to black rats in captivity, whereas the intermediate sized seeds of both native and
non-native species tended to have the lowest survival (Chapters 4 and 5). In a recent
review of New Zealand studies, Grant-Hoffman and Barboza (2010) found that relatively
small seeds were most commonly reported as eaten by introduced rats. In recent field
trials in Hawaiian dry forest (Chimera and Drake, in revision), and in cloud forest on
Lord Howe Island (Auld et al. 2010), diaspore and seed size did not appear to influence
removal by introduced rodents.
Hypothesis 5: Seed survival is inversely correlated with seed size, and the upper limit for
survival is ca. 2.5 mm.
Findings: This hypothesis was not supported. When seeds of 25 of the most
problematic invasive plant species were offered to wild black rats in captivity, rats
consumed proportionally more seed mass of the smaller fruits and seeds than the larger
ones; however fruit and seed size did not predict seed survival (Chapter 5). One possible
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explanation for the proportionally greater consumption of large fruits and seeds relative
to smaller fruits and seeds by black rats may be related to the reduced time to satiation
when large seeds are eaten (Janzen 1971). As described in the previous hypothesis, most
of the intermediate-sized seeds that I offered to black rats tended to have higher predation
(lower survival) relative to the largest and smallest seeds.
There are three ways in which a seed can be dispersed by black rats, and seed size
is a critical factor in one of these types of dispersal (gut passage) but less important for
the others (discarded seeds or chewed seeds). Because it is a common behavior of black
rats to move food items upon collection, they can disperse seeds of a wide range of sizes
if they do not destroy the seed or if a fraction of the seeds in a multi-seeded fruit are not
destroyed. Similarly, chewed seeds can be dispersed by black rats as long as the chewed
seed retains viability. Alternatively, seeds may be dispersed by gut passage if they are
small enough to pass intact through the rat‘s digestive tract without losing viability. I
found evidence for all three types of seed dispersal in my studies with black rats (Chapter
4, 5). The possibility of seed dispersal by black rats in Hawaii does not appear to be
limited by seed size.
The upper size limit for seed survival following gut passage in black rats was ca.
2.4 mm in New Zealand captive-feeding trials (Williams et al. 2000). However, my
findings for black rats in Hawaii revealed that the upper size limit for seed survival is 1.52.0 mm (Chapter 5), as evidenced by captive-feeding trials, using 25 invasive species and
eight native Hawaiian species, and germination trials with seeds that were passed through
the rat‘s gut (Chapter 4, 5). Additional trials with Cyanea superba (unpublished data)
reveal that the seed size threshold separating gut passage from seed predation for black
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rats is ca. 1.5-1.7 mm; therefore, species with seeds that are consumed by black rats with
seeds > 1.5 mm are more likely to be destroyed than seeds ≤ 1.5 mm. Basing such a
threshold on seed mass is less useful than seed length for the tested species because the
2.1 mm seeds of Schefflera actinophylla weigh 0.2 mg and did not survive black rat
consumption, whereas Rubus rosifolius seeds (1.5 mm length) survived black rat
consumption despite the 0.5 mg larger seeds than those of S. actinophylla. Additional
factors other than seed length and mass may also affect a seed‘s propensity to survive
consumption by rats (e.g., seed coat characteristics), but these have yet to be studied.
Future research involving introduced rodent effects on island flora
Certain aspects of the seed fate of many species that black rats interact with have
now been determined using captive feeding trials (ca. 85 species to date from my
research), yet the majority of the species in Hawaii alone have not been tested and very
few trials exist that measure species vulnerability (either plants or animals) to Pacific rats
and the house mouse. Some data exist on the extent to which rodents affect seeds
through pre-dispersal seed predation (e.g., Psidium cattleianum, Diospyros spp.;
unpublished data), but it is largely unknown which of the rodent species is responsible for
this; nor is much known about the extent to which species suffer pre-dispersal vs. postdispersal seed predation. Almost certainly, the effects of introduced rodents are
influenced by many features of their habitat, including the plant species with which they
interact. Future study on seedling establishment and survival within microhabitats visited
by rodents (e.g., husking stations, defecation sites) would help determine the ultimate fate
of seeds that were discarded, partially chewed, or defecated by rodents.
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The future research needs for improving our understanding and management of
rodent impacts on islands could usefully focus on the degree to which seed predation and
dispersal by each of the introduced rodents affects plant recruitment. Some of this
research has begun at KHI, and we are finding that reducing the abundance of introduced
rodents (mainly rats) reduces seed predation on native (Diospyros spp.) and endangered
(Cyanea superba) species (unpublished data; Mosher et al. 2010). Additionally, rodent
reduction at KHI appears to have stimulated seedling recruitment of Diospyros spp.
relative to a nearby forest where rodents were not reduced (unpublished data). Rodent
removal experiments where both experimental sites and nearby control sites are
monitored simultaneously are necessary to provide resolution to the critical question that
many of us commonly ask: To what extent are introduced rodents altering plant
recruitment and community structure in Hawaii?
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APPENDIX A. Densities (No. indiv./ha) of the 35 woody plant species with stems ≥ 1
cm dbh (measured at 1.3 m above ground) recorded in 10 15 x 15 m plots at Kahanahaiki,
Oahu, in August 2008. An asterisk represents a non-native species. Cyanea superba,
Cyrtandra dentata, and Flueggea neowawraea are federally endangered species, and
Pteralyxia macrocarpa is a species of concern. Both the red and yellow varieties of
Psidium cattleianum are represented at the site. Species ordering based on density.
Species
Family
Life form
Psidium cattleianum*
Myrtaceae
Tree
Density
(No./ha)
1640
Diospyros hillebrandii
Ebenaceae
Tree
631
Psydrax odorata
Rubiaceae
Tree
489
Sapindaceae
Tree
324
Anacardiaceae
Tree
271
Rubiaceae
Tree
187
Amaranthaceae
Tree
147
Pouteria sandwicensis
Sapotaceae
Tree
142
Kadua affinis
Rubiaceae
Tree
142
Aleurites moluccana*
Euphorbiaceae
Tree
129
Hibiscus arnottianus
Malvaceae
Tree
98
Euphorbiaceae
Tree
98
Cordyline fruticosa*
Agavaceae
Shrub
71
Pisonia umbellifera
Nyctaginaceae
Tree
71
Rubiaceae
Tree
49
Pandanaceae
Liana
44
Oleaceae
Tree
40
Pipturus albidus
Urticaceae
Tree
36
Psidium guajava*
Myrtaceae
Tree
31
Diospyros sandwicensis
Ebenaceae
Tree
31
Coprosma foliosa
Rubiaceae
Shrub
22
Sapindus oahuensis
Schinus terebinthifolius*
Psychotria mariniana
Charpentiera tomentosa
Antidesma platyphyllum
Psychotria hathewayi
Freycinetia arborea
Nestegis sandwicensis
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Species
Family
Life form
Pisonia brunoniana
Nyctaginaceae
Tree
Density
(No./ha)
22
Bobea sandwicensis
Rubiaceae
Tree
22
Xylosma hawaiiensis
Flacourtiaceae
Tree
22
Myrsine lessertiana
Myrsinaceae
Tree
18
Cyanea superba
Campanulaceae
Tree
18
Flueggea neowawraea
Euphorbiaceae
Tree
18
Rubiaceae
Tree
13
Pteralyxia macrocarpa
Apocynaceae
Tree
9
Buddleia asiatica*
Buddleiaceae
Shrub
4
Rauvolfia sandwicensis
Apocynaceae
Tree
4
Cyrtandra dentata
Gesneriaceae
Shrub
4
Grevillea robusta*
Proteaceae
Tree
4
Metrosideros polymorpha
Myrtaceae
Tree
4
Syzygium cumini*
Myrtaceae
Tree
4
Morinda trimera
185
APPENDIX B. Mean ± SE seedling (individuals ≤ 50 cm tall) densities of woody
species at Kahanahaiki, Oahu. An asterisk represents a non-native species. A total of 80
seedling plots (each 1 x 2 m) were sampled across 10 replicated permanent plots (each 15
x 15 m) in August 2009. Values shown below are N = 10 (where eight seedling plot
measures were combined for each permanent plot).
Species
Family
Life form
Seedling density
(No./m2)
Aleurites moluccana*
Euphorbiaceae
Tree
0.16 ± 0.14
Alyxia stellata
Apocynaceae
Shrub/liana
0.49 ± 0.37
Antidesma platyphyllum
Euphorbiaceae
Tree
0.01 ± 0.01
Clidemia hirta*
Melastomataceae
Shrub
0.21 ± 0.08
Coprosma foliosa
Rubiaceae
Shrub
0.02 ± 0.02
Diospyros hillebrandii
Ebenaceae
Tree
0.03 ± 0.02
Diospyros sandwicensis
Ebenaceae
Tree
0.01 ± 0.01
Kadua affinis
Rubiaceae
Tree
0.01 ± 0.01
Nestegis sandwicensis
Oleaceae
Tree
0.03 ± 0.03
Pisonia brunoniana
Nyctaginaceae
Tree
0.02 ± 0.01
Pouteria sandwicensis
Sapotaceae
Tree
0.01 ± 0.01
Psidium cattleianum*
Myrtaceae
Tree
0.46 ± 0.30
Psydrax odorata
Rubiaceae
Tree
0.01 ± 0.01
Sapindus oahuensis
Sapindaceae
Tree
0.01 ± 0.01
Schinus terebinthifolius*
Anacardiaceae
Tree
0.04 ± 0.03
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APPENDIX C. Mean flower, fruit, and seed rain (No./m2/day) measured at Kahanahaiki,
Oahu, from 20 June 2007-6 July 2010. Each value represents the average of 72 sampling
periods (separated by ca. 2 weeks) during the three-year monitoring. At each sampling
period, flowers, fruits, and seeds were identified in 48 seed buckets that were placed in 12
15 x 15 m plots (four buckets per plot). Several species‘ reproductive material was too
small to identify in the field and therefore was not considered (e.g., most seeds < 1.5 mm
unless fruit was present).
Speciesa
Family
Aleurites moluccana*
Euphorbiaceae
Alyxia stellata
Apocynaceae
0.01
< 0.01
< 0.01
Antidesma platyphyllum
Euphorbiaceae
0
< 0.01
< 0.01
Buddleia asiatica*
Buddleiaceae
0.02
0.59
0
Canavalia galeata
Fabaceae
< 0.01
0
0
Carex hawaiiensis
Cyperaceae
0
< 0.01
< 0.01
Charpentiera tomentosa
Amaranthaceae
< 0.01
0
0
Clidemia hirta*
Melastomataceae
0.01
0.04
0.42
Coprosma foliosa
Rubiaceae
0
0
< 0.01
Cordyline fruticosa*
Agavaceae
< 0.01
< 0.01
< 0.01
Diospyros spp.
Ebenaceae
0.32
0.03
< 0.01
Dodonaea viscosa
Sapindaceae
0
< 0.01
< 0.01
Grevillea robusta*
Proteaceae
0.05
< 0.01
0.01
Hibiscus arnottianus
Malvaceae
< 0.01
0
0
Kadua affinis
Rubiaceae
0.02
0
0
Korthalsella degeneri
Viscaceae
0
1.09
1.09
Metrosideros polymorpha
Myrtaceae
0.01
0.05
0
Myrsine lessertiana
Myrsinaceae
< 0.01
0
0
Nestegis sandwicensis
Oleaceae
0
< 0.01
< 0.01
Oplismenus hirtellus*
Poaceae
0
0.02
0.02
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Flowers
Fruits
Seeds
(No./m2/d) (No./m2/d) (No./m2/d)
5.08
< 0.01
< 0.01
Speciesa
Family
Paspalum conjugatum*
Poaceae
Passiflora spp.*
Passifloraceae
Pipturus albidus
Urticaceae
Pisonia spp.
Nyctaginaceae
Pouteria sandwicensis
Flowers
Fruits
Seeds
(No./m2/d) (No./m2/d) (No./m2/d)
0
< 0.01
< 0.01
0
0
< 0.01
0.02
< 0.01
0
< 0.01
< 0.01
< 0.01
Sapotaceae
0.26
< 0.01
< 0.01
Psidium cattleianum*
Myrtaceae
2.15
0.43
6.14
Psidium guajava*
Myrtaceae
0.01
< 0.01
< 0.01
Psychotria spp.
Rubiaceae
< 0.01
< 0.01
< 0.01
Psydrax odorata
Rubiaceae
0.02
0.04
0.05
Rubus rosifolius*
Rosaceae
< 0.01
0
0.10
Sapindus oahuensis
Sapindaceae
0.22
0.03
0.03
Schinus terebinthifolius*
Anacardiaceae
5.39
2.58
2.58
Xylosma hawaiiensis
Flacourtiaceae
0.05
0
0
0.05
0
< 0.01
Unknown
a
Candidate species for the following identified genera include: 1) Diospyros: D.
hillebrandii, D. sandwicensis, 2) Passiflora: P. edulis, P. suberosa, 3) Psychotria: P.
hathewayi, P. mariniana.
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APPENDIX D. Frequency of food types in scat from mongoose (Herpestes javanicus; N
= 31) and cat (Felis catus; N = 13) collected opportunistically by Steve Mosher at
Kahanahaiki, Oahu, during 2005-2007. Samples were kept frozen until extraction.
Extracted contents were passed through a 0.4 mm mesh sieve, followed by microscopic
analysis (10x-20x magnification) using methods described for rodent stomach contents in
Chapter 3.
Rat
Mouse Reptile
Plant
Arthropod
Mollusk
Bird
Mongoose
26%
77%
87%
84%
97%
10%
26%
Cat
100%
100%
31%
92%
100%
0%
23%
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APPENDIX E. Captive feeding trials involving rodent species (N = 11 black rats or N =
2 house mice) offered one of two treatments: 1) a Deroceras laeva slug, or 2) a
Deroceras laeva slug simultaneously with ca. 2 g of rodent chow. Each trial lasted 24
hours. All three rats that were offered rodent chow and a slug ate all the rodent chow and
at least part of the slug. Cage dimensions and capture locations of the rodents used in
these trials were identical to those described in Chapter 5. All slugs were captured in the
Waianae Mountains, Oahu.
Rodent species
Treatment
Killed slug?
Ate slug?
Approx.
amount of slug
eaten (%)
Black rat
Chow + slug
Yes
Yes
100
Black rat
Chow + slug
Yes
Yes
75
Black rat
Chow + slug
Yes
Yes
75
Black rat
Slug
Yes
Yes
20
Black rat
Slug
Yes
Yes
100
Black rat
Slug
Yes
Yes
100
Black rat
Slug
Yes
Yes
15
Black rat
Slug
Yes
Yes
85
Black rat
Slug
Yes
Yes
100
Black rat
Slug
Yes
Yes
95
Black rat
Slug
Yes
Yes
80
House mouse
Slug
No
No
0
House mouse
Slug
No
No
0
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APPENDIX F. Captive feeding trials involving rodent species (N = 11 black rats or N =
2 house mice) offered one Limax maximus slug. Each trial lasted 24 hours. Cage
dimensions and capture locations of the rodents used in these trials were identical to those
described in Chapter 5. All slugs were captured in the Waianae Mountains, Oahu.
Rodent species
Treatment
Killed slug?
Ate slug?
Approx.
amount of slug
eaten (%)
Black rat
Slug
Yes
Yes
95
Black rat
Slug
Yes
Yes
100
Black rat
Slug
Yes
Yes
85
Black rat
Slug
Yes
No
0
Black rat
Slug
Yes
No
80
Black rat
Slug
Yes
No
0
Black rat
Slug
Yes
No
0
Black rat
Slug
Yes
No
0
Black rat
Slug
Yes
Yes
7
Black rat
Slug
Yes
No
0
Black rat
Slug
Yes
Yes
98
House mouse
Slug
No
No
0
House mouse
Slug
No
No
0
191
APPENDIX G. Estimated ages of black rats at Kahanahaiki, Oahu. Ages were
calculated by adding three months (i.e., ca. time to maturity) to the period from first
capture to death by kill-traps. Deaths occurred May 5-15, 2009, during the trap-out. The
distance moved, and weight change (+ or – if gained or lost, respectively), were
calculated by the difference from trapping locations and mass when first caught and at
death. A distance of zero indicates the rat was captured (and killed) in the same location
as initially captured.
Ear tag
#
Sex
Date of first
capture (m-d-y)
Minimum age at
death (months)
Distance
moved (m)
Weight
change (g)
831
Male
02-12-09
6
0
+1
630
Female
08-29-08
11
40
+3
858
Male
04-09-09
4
25
+11
698
Male
10-10-08
10
25
-14
866
Male
04-12-09
4
25
+7
653
Male
08-31-08
11
150
+47
377
Male
01-04-08
19
200
+72
829
Male
02-12-09
6
25
+3
356
Male
01-04-08
19
10
+60
644
Female
08-29-08
11
0
-4
693
Male
10-10-08
10
0
+50
447
Male
02-28-08
18
15
+53
408
Female
01-04-08
19
50
+36
863
Male
04-10-09
4
0
+3
531
Female
04-24-08
16
175
+21
855
Male
04-09-09
4
20
+7
379
Female
01-04-08
19
25
+54
786
Female
12-07-08
6
0
+28
192